In general, a CdTe system compound semiconductor single crystal has the property of being high resistivity when Group 13 (3B) elements such as Al(aluminum), Ga(gallium), and In(indium), Group 17 (7B) elements such as F(fluorine), Cl(chlorine), Br(bromine), and I(iodine), Group 14 (4B) elements such as Ge(germanium) and Sn(tin), or transition metal element such as V(vanadium) is included in a small amount. Further, a CdTe system compound semiconductor single crystal has the property of exhibiting n-type conductivity when the CdTe system compound semiconductor single crystal is doped with sufficient amount of Group 13 (3B) elements or Group 17 (7B) elements. A high resistance CdTe system compound semiconductor single crystal which is obtained in such way is used as a substrate for a photorefractive element, an electro-optic element (EO element), and a radiography sensing element. In case where the CdTe system compound semiconductor single crystal is used for the above described purpose, it is known that the higher the resistivity of the CdTe system compound semiconductor single crystal substrate is, the more device properties enhance, and the resistivity is preferred to be more or equal to 1×108 Ωcm.
For example, in Patent Document 1, a CdTe single crystal in which the concentration of Ga as a dopant is set to be in the range of 5×10l6 to 5×1018 cm−3 and the concentration of other impurity atoms is set to be below or equal to 7×1014 cm−3 in order to obtain a high resistance photorefractive crystal is disclosed. Here, Ga is used as a dopant because when In or V is used as a dopant, high concentration doping is needed in order to realize high resistivity and the crystalline is poor. According to the above described technique, high resistivity of more or equal to 1×108 Ωcm can be realized in a CdTe single crystal.
Further, for example, in Patent Document 2, a CdTe single crystal in which the concentration of Cl in a crystal is in the range of 0.1 to 5. 0 ppmwt and the resistivity at room temperature is 1.0×109 Ωcm is disclosed.
Meanwhile, a CdTe system compound semiconductor single crystal is widely used as a substrate for an infrared sensor for an approximate 2 to 20 μm band. In particular, a p-n junction diode in which p-type and n-type HgCdTe epitaxial layers are formed on a CdZnTe substrate is used. Concerning this CdZnTe substrate for an optical device, high quality p-type and n-type HgCdTe epitaxial layers on the substrate are important, and the substrate which does not have an adverse influence on the epitaxial layers is more important than the electrical properties of the substrate. However, there is a concern that impurities in the substrate diffuse to the epitaxial layers and have an adverse effect on the p-n characteristics. Therefore, the technique of doping with a small amount of impurities in order to obtain a high resistance substrate for a radiation detector and the like as described above is not preferred.
Patent Document 1 : Toku-kai-hei 5-70298
Patent Document 2: Toku-kai-hei 11-228299